The present invention generally relates to gas turbine engines and, more specifically, to an aircraft Auxiliary Power Unit (APU) with an oil cooled generator having an oil tank integral therewith providing gravity scavenging of oil from the generator.
APUs are generally small gas turbine engines, often mounted in the aft tailcone section of the aircraft with an exhaust directed out of the aft of the tailcone section. The APU typically provides electrical and pneumatic power to the aircraft for main engine starting and passenger compartment cooling. The APU may supply pneumatic and electric power during flight as well as provide emergency power during flight.
A typical APU has a power section with a combustor proximate an aft end and an air compressor proximate a forward end. The combustor receives compressed air from the air compressor and fuel from a fuel supply source in order to generate a high pressure combustion product for rotating a turbine shaft. A gearbox is typically mounted to the forward side of the power section having the air compressor and reduces the rotational speed of the turbine shaft by gear reduction down to the required speed to properly operate the generator and other components mounted to the gearbox. Other components that may be driven by the gearbox may include such components as an air oil separator, oil pump and fuel pump assemblies, and other associated components. A starter may also be mounted to the gearbox for starting the power section. Lubricating and/or cooling oil is typically supplied to components disposed within or associated with the APU such as the gearbox, generator, and power section. Typically, the generator has the greatest demand for such oil since the generator is typically an oil cooled generator requiring oil to cool the stator and other electrical components as well as to lubricate bearings and splines.
Referring to FIGS. 1A and 1B, there is shown an APU 100 of the prior art having an oil tank 108 integral with a gearbox 104. Gearbox 104 is mounted to the cooler end of a turbine engine 106 opposite an exhaust 122. Gearbox 104 typically has a housing 101 comprised of cast aluminum and has a lower portion serving as oil tank 108 where the oil is pumped to the various components of APU 100 and returned thereto. Gearbox 104 has a generator 102 mounted to the forward end thereof, opposite turbine engine 106, and is axially parallel with a turbine shaft substantially extending turbine engine 106 along an engine centerline 120. As shown in FIG. 1B, gearbox 104 has a relatively small depth 103 along engine centerline 120. Therefore, gearbox 104 extends well below and about the gears therein to accommodate the volume of cooling and lubricating oil needed and to minimize oil churning by the gears. APU 100 is typically installed in the tailcone section of an aircraft with exhaust 122 exiting an aft portion of the aft tailcone section.
Due to the location and configuration of generator 102, the cooling and lubricating oil must be mechanically scavenged (pumped against the force of gravity) by an oil pump, shown as a component of an oil and fuel pump assembly 116, and returned to oil tank 108 located in the bottom of gearbox 104. When APU 100 is installed in an aircraft, various flight attitudes of the aircraft complicates the scavenging process and may necessitate the use of separate cored oil passages and pump elements to minimize the risk of oil flooding either side of a generator rotor. Failure to scavenge the oil from the generator rotor may result in excess heat generation and mechanical failure of the generator 102. A mechanically failed generator may contaminate the engine oil system and result in complete failure of the APU 100.
Gearbox 104 has components in addition to generator 102 mounted to the forward side thereof which may also be powered by the power section gearbox 104. These components may include, for example, an air/oil separator 112, a starter 114, and oil pump and fuel pump assembly 116. Additional and different components may be mounted to and powered by gearbox 104.
Gearbox housing 101 is typically comprised of cast aluminum and has a lower portion that serves as oil tank 108 where the oil is pumped to the various components of APU 100 and returned thereto. In order to accommodate this oil, gearbox housing 101 extends well around and below the gears therein to minimize oil pickup by the rotating gears. Generator 102 is typically mounted to the forward side of gearbox 104, opposite a power section having turbine engine 106, and axially aligned with a turbine shaft substantially longitudinally extending the turbine engine 106. Due to the location of generator 102 on the extreme forward and frequently lower end of APU 100, the oil must be mechanically scavenged (pumped against the force of gravity) by the oil pump and returned to oil tank 108, located in the bottom of gearbox 104. Oil needs to be scavenged from both ends of the generator rotor and returned to the oil tank. This requires complex lubrication systems to make it function properly. Failure to collect the oil from both ends of generator 102 can result in oil being churned by the rotor, excess generation of heat, and even catastrophic generator failure. An aircraft in various flight attitudes complicates this scavenging process and may necessitate the use of separate cored oil passages and pump elements to minimize the risk of oil flooding either side of the generator rotor and generator failure. A mechanically failed generator can produce debris. When scavenged by the pump and returned to oil tank 108 the debris can be churned up by the rotating gears during extreme flight attitudes. This debris in the gear train and bearings can result in severe damage and complete failure of APU 100.
In addition to the aforementioned problems associated with current designs of the gearbox having an integral oil tank, other problems have been found. A gearbox with an integral oil tank may limit configurations of components mounted thereto and the locations on the gearbox where they may be mounted to keep the gear train from churning the oil stored in the tank below. These current design practices necessitate high maintenance costs since the entire APU typically must be removed from the aircraft and the gearbox needs to be removed from the APU to enable maintenance and cleaning of the engine oil tank. Also, fire testing of the gearbox may be required to prove it meets the fire proof requirement as aluminum is not recognized as providing a fireproof oil tank. Additionally, the weight and cost of the oil tank may be greater than needed.
As can be seen, there is a need for an APU having a reduced risk of failure, smaller gearbox, lower associated maintenance costs, less weight and cost.